In assembling ultra high vacuum (UHV) systems operating at pressures below approximately 10-6 torr, seals and closures have presented challenging problems. For such systems, rubbers and elastomers are not suitable as seals as they are permeable to gases to an extent such that very low base pressures cannot be attained and such seals typically cannot survive the high temperatures often necessary to bake out vacuum systems. When frequent separation or proximity of heat sensitive materials makes welding impractical, plastically deformed metal seals are satisfactory. The seals, however, require very rigid flanges and many closely spaced bolts to accomplish a sufficiently tight seal, and assembly and disassembly are time consuming. Nevertheless, plastically deformed metal seals such as gold wire and flat copper washers are the most reliable seals and are used almost universally in UHV work. Many styles have been developed, from laboratory fabricated special seals to commercially available standardized seals.
Some typical seals are illustrated in FIG. 1a to FIG. 1e. 
FIG. 1a shows a crushed wire ring gasket configuration, with a gasket 20, an upper flange 30, a lower flange 10 and a dead space 1. Crushed wire rings can be made of gold, copper, or aluminum. These gaskets are generally made from round wire with a desired diameter cut to the length of the gasket's mean circumference, then formed into a circle and welded. They provide positive pressures. Since they only begin with a line of contact, they have a high local seating stress at low bolt loads. The contact faces increase in flowing into flange faces.
FIG. 1b shows a crushed step seal with flat gasket configuration, with a gasket 20, an upper flange 30, a lower flange 10 and a dead space 1, wherein the soft metal gasket 20 is crushed and deformed in a groove of the two flat flange members by a bolt down force.
FIG. 1c shows a coined gasket seal, with a gasket 20, an upper flange 30, a lower flange 10 and a dead space 1 wherein a rectangular shaped gasket crushed in a groove by bolt down force.
FIG. 1d shows a knife edge seal with a gasket 20, an upper flange 30, a lower flange 10 and a dead space 1, wherein the two flanges with V-shaped ridges facing each other and a soft metal gasket in between, held together by a clamp or bolt. The V-shaped ridge is filled in with the material as the gasket is deformed.
FIGS. 1e and 1f show a Conflat® brand metallic vacuum sealing flange seal by Varian Associates with a gasket 20, an upper flange 30, a lower flange 10 and a dead space 1, wherein a soft metal gasket captured in a rigid structure which deforms the gasket. Long bake out times at high temperatures relieve internal stresses and the force on the seal, allowing the joint to be leak-tight as differential thermal expansion is limited.
Cryofit® brand heat-shrinkable hydraulic couplings by Raychem Corporation utilize Nitinol, a shape-memory alloy. The Cryofit® connector is essentially a sleeve of Nitinol, having internal seal ridges in series, which is bored in the austenitic phase to a diameter less then the outside diameter of pipes to be joined, then chilled and transformed to martensite, and mechanically expanded to a diameter greater than the outside diameter of pipes to be joined. The connector can be slipped over the pipes, heated and transformed to austenite, whereby a hoop stress presses the series of seal ridges into the outside surface of the pipes making an excellent pipe and hydraulic connection.
Helicoflex® brand metal gaskets by Carbone-Lorraine Industries Corporation utilize a shape-memory alloy to seal cavities, as disclosed in U.S. Pat. No. 4,445,694.
All these sealing forces, external normal force bolt down or internal pressure, are applied to deform the metal gasket. The deformed metal gasket blocks the channels that connect the confined chamber and outside environment. Thereafter the chamber can be pumped down to the desired ultrahigh vacuum (UHV) with proper equipment.
Obara et al. in U.S. Pat. No. 4,988,130 discloses a formula to estimate how many bolts are needed to affix a pair of flanges of thickness t using a copper gasket as a seal for a pipe end of a plasma vacuum vessel. These flanges each have a ring-shaped knife edge. However, Obara's suggestion is for a large round-shaped pipe end with M8 bolts, and is not suitable for a smaller size vessel with multiple openings to be sealed with thinner M5 bolts and limited space to affix the bolts.
The present invention provides a metal gasket that is easily deformed with fewer bolts within a limited working space, yet still provides the same ability to reach a desired ultrahigh vacuum (UHV) level.